No idea! I have the same problem, up every night sometimes as much as 15 times in one night, also waking up after only 5 hours of sleep and not being able to sleep for about a couple hours afterward (mind you I am very very tired in those couple hours). I heard somewhere that it's your body preparing itself for when the baby comes(waking you up every night and all), but that's not definite I could be wrong?

Hope it helps.